static int MinAvgTwoSlice(int[] a)
{
    var idx = 0;
    double min = (a[0] + a[1]) / 2.0;

    for (int i = 0; i < a.Length - 1; i++)
    {
        double cur = (a[i] + a[i + 1]) / 2.0;
        if (i + 2 < a.Length)
        {
            double three = (a[i] + a[i + 1] + a[i + 2]) / 3.0;
            cur = cur < three ? cur : three;
        }
        if (cur < min)
        {
            min = cur;
            idx = i;
        }
    }

    return idx;
}